Proposed involvement of adipocyte glyceroneogenesis and phosphoenolpyruvate carboxykinase in the metabolic syndrome

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

Transcriptomic signature of fasting in human adipose tissue

Little is known about gene regulation by fasting in human adipose tissue. Accordingly, the objective of this study was to investigate the effects of fasting on adipose tissue gene expression in humans. To that end, subcutaneous adipose tissue biopsies were collected from 11 volunteers 2 and 26 h after consumption of a standardized meal. For comparison, epididymal adipose tissue was collected from C57Bl/6J mice in the ab libitum-fed state and after a 16 h fast. The timing of sampling adipose tissue roughly corresponds with the near depletion of liver glycogen. Transcriptome analysis was carried out using Affymetrix microarrays. We found that, 1) fasting downregulated numerous metabolic pathways in human adipose tissue, including triglyceride and fatty acid synthesis, glycolysis and glycogen synthesis, TCA cycle, oxidative phosphorylation, mitochondrial translation, and insulin signaling; 2) fasting downregulated genes involved in proteasomal degradation in human adipose tissue; 3) fasting had much less pronounced effects on the adipose tissue transcriptome in humans than mice; 4) although major overlap in fasting-induced gene regulation was observed between human and mouse adipose tissue, many genes were differentially regulated in the two species, including genes involved in insulin signaling (PRKAG2, PFKFB3), PPAR signaling (PPARG, ACSL1, HMGCS2, SLC22A5, ACOT1), glycogen metabolism (PCK1, PYGB), and lipid droplets (PLIN1, PNPLA2, CIDEA, CIDEC). In conclusion, although numerous genes and pathways are regulated similarly by fasting in human and mouse adipose tissue, many genes show very distinct responses to fasting in humans and mice. Our data provide a useful resource to study adipose tissue function during fasting.

Nuclear receptor crosstalk - defining the mechanisms for therapeutic innovation

Nuclear receptor crosstalk can be defined as the interplay between different nuclear receptors or between their overlapping signalling pathways. A subset of nuclear receptors (such as PPARs and RARs) engage in the formation of well-characterized 'typical' heterodimers with RXR. 'Atypical' heterodimers (such as GR with PPARs, or PPAR with ERR) might form a novel class of physical complexes that might be more transient in nature. These heterodimers might harbour strong transcriptional flexibility, with no strict need for DNA binding of both partners. Direct crosstalk could stem from a pairwise physical association between atypical nuclear receptor heterodimers, either via pre-existing interaction pairs or via interactions that are newly induced with small molecules; such crosstalk might constitute an uncharted space to target nuclear receptor physiological and/or pathophysiological actions. In this Review, we discuss the emerging aspects of crosstalk in the nuclear receptor field and present various mechanistic crosstalk modes with examples that support applicability of the atypical heterodimer concept. Stabilization or disruption, in a context-dependent or cell type-dependent manner, of these more transient heterodimers is expected to fuel unprecedented translational approaches to yield novel therapeutic agents to treat major human diseases with higher precision.

Do Benefits Outweigh Risks for Corticosteroid Therapy in Acute Exacerbation of Chronic Obstructive Pulmonary Disease in People with Diabetes Mellitus?

Chronic obstructive pulmonary disease (COPD) and diabetes mellitus (DM) are chronic health conditions with significant impacts on quality and extent of life. People with COPD and DM appear to have worse outcomes in each of the comorbid conditions. Treatment with corticosteroids in acute exacerbation of COPD (AECOPD) has been shown to reduce treatment failure and exacerbation relapse, and to shorten length of hospital stay, but not to affect the inexorable gradual worsening of lung function. Treatment with corticosteroids can lead to a wide spectrum of side effects and complications, including worsening hyperglycemia and deterioration of diabetes control in those with pre-existing DM. The relationship between COPD and DM is rather complex and accumulating evidence indicates a distinct phenotype of the comorbid state. Several randomized controlled trials on corticosteroid treatment in AECOPD excluded people with DM or did not report on outcomes in this subgroup. As such, the perceived benefits of corticosteroids in AECOPD in people with DM have not been validated. In people with COPD and DM, the detrimental side effects of corticosteroids are guaranteed, while the benefits are not confirmed and only presumed based on extrapolation from the general COPD population. Therefore, the potential for harm when prescribing corticosteroids for AECOPD in people with DM cannot be excluded.

High released lactate by epicardial fat from coronary artery disease patients is reduced by dapagliflozin treatment

BACKGROUND AND AIMS

Dapagliflozin, a sodium-glucose co-transporter 2 inhibitor, improves glucose uptake by epicardial adipose tissue (EAT). However, its metabolism might raise the lactate production and acidosis under hypoxia conditions, i.e. coronary artery disease (CAD), or lipogenesis and, in consequence, expand adipose tissue. Since lactate secreted by adipose tissue is correlated with tissue stress and inflammation, our aim was to study glucose metabolism by epicardial fat in CAD and its regulation by dapagliflozin.

METHODS

Paired EAT and subcutaneous adipose tissue (SAT) biopsies from 49 patients who underwent open-heart surgery were cultured and split into three equal pieces, some treated with and others without dapagliflozin at 10 or 100 μM for 6 h. Anaerobic glucose metabolites were measured in supernatants of fat pads, and acidosis on adipogenesis-induced primary culture cells was analysed by colorimetric or fluorescence assays. Gene expression levels were assessed by real-time polymerase chain reaction.

RESULTS

Our results showed that dapagliflozin reduced the released lactate and acidosis in epicardial fat (p < 0.05) without changes in lipid storage-involved genes. In addition, this drug induced gene expression levels of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α), a mitochondrial biogenesis-involved gene in both EAT and SAT (p < 0.05). After splitting the population regarding the presence of CAD, we observed higher lactate production in EAT from these patients (2.46 [1.75-3.47] mM), which was reduced after treatment with dapagliflozin 100 μM (1.99 [1.08-2.99] mM, p < 0.01).

CONCLUSIONS

Dapagliflozin improved glucose metabolism without lipogenesis-involved gene regulation or lactate production, mainly in patients with CAD. These results suggest an improvement of glucose oxidation metabolism that can contribute to cardiovascular benefits.

Anti-Diabetogenic Properties of Mineralocorticoid Receptor Antagonists: Implications for Enhanced Safety and Efficacy of Post-Transplantation Pharmacotherapies

Widespread usage of the calcineurin inhibitors tacrolimus and cyclosporine A as post-transplantation immunosuppressive agents is fraught with severe nephrotoxic and diabetogenic side effects. More recently, tapering of calcineurin inhibitor-based immunotherapies with concurrent administration of the mammalian target of rapamycin (mTOR) inhibitors sirolimus and everolimus has been employed within pharmacological regimens designed to achieve better safety and efficacy for preservation of allograft kidney function. Collected preclinical data and recent clinical study, however, indicate that usage of calcineurin inhibitors and/or mTOR blockers as immunosuppressive agents promotes equivalent diabetogenic side effects. Based on a wealth of validating preclinical studies, we contend that the favorable metabolic effects of mineralocorticoid receptor antagonists, such as spironolactone, support their inclusion in novel immunosuppressive strategies to inhibit new onset type II diabetic symptoms in post-transplantation patient populations.

Which is the proper reference tissue for measuring the change in FDG PET metabolic volume of cardiac sarcoidosis before and after steroid therapy?

Background

Cardiac sarcoidosis (CS) is a rare but potentially life-threatening disease that causes conduction disturbance, systolic dysfunction, and, most notably, sudden cardiac death. ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) plays important roles not only in diagnosing CS but also in evaluating the effects of anti-inflammatory therapy. A volume-based analysis of parameters measured by FDG PET, so-called cardiac metabolic volume (CMV), has emerged as a new assessment tool. CMV is measured as the volume within the boundary determined by a reference tissue such as the liver and the blood pool uptake. However, there is a possibility that oral steroid therapy could lead to variations of the liver and the blood pool uptake. Here, we attempted to evaluate the steroid effects on the liver and the blood pool uptake.

A total of 38 CS patients who underwent FDG PET/CT before and during steroid therapy were retrospectively enrolled. Volumes of interest (VOIs) were placed in the right lobe of the liver and descending aorta (DA). The maximum standardized uptake value (SUVmax), SUVmean, and SUVpeak of the liver and DA were compared between time points before and during steroid therapy.

Results

The SUVmax, SUVmean, and SUVpeak of the liver during steroid therapy significantly increased from the time point before the therapy (SUVmax 3.5 ± 0.4 vs. 3.8 ± 0.6, p = 0.014; SUVmean 2.7 ± 0.3 vs. 3.0 ± 0.5, p = 0.0065; SUVpeak 3.0 ± 0.4 vs. 3.4 ± 0.6, p = 0.006). However, the SUVmax, SUVmean, and SUVpeak in the DA did not significantly change (SUVmax 2.2 ± 0.3 vs. 2.2 ± 0.4, p = 0.46; SUVmean 1.9 ± 0.3 vs. 2.0 ± 0.4, p = 0.56; SUVpeak 2.0 ± 0.3 vs. 2.0 ± 0.3, p = 0.70).

Conclusions

We measured FDG uptake in the liver and blood pool before and during steroid therapy. Steroid therapy increased the liver uptake but not the blood pool uptake. Our findings suggested that the DA uptake is a more suitable threshold than liver uptake to evaluate therapeutic effects using volume-based analysis of cardiac FDG PET.

Transcriptomic Analyses of Adipocyte Differentiation From Human Mesenchymal Stromal-Cells (MSC)

Adipogenesis is a physiological process required for fat-tissue development, mainly involved in regulating the organism energetic-state. Abnormal distribution-changes and dysfunctions in such tissue are associated to different pathologies. Adipocytes are generated from progenitor cells, via a complex differentiating process not yet well understood. Therefore, we investigated differential mRNA and miRNA expression patterns of human mesenchymal stromal-cells (MSC) induced and not induced to differentiate into adipocytes by next (second)-generation sequencing. A total of 2,866 differentially expressed genes (101 encoding miRNA) were identified, with 705 (46 encoding miRNA) being upregulated in adipogenesis. They were related to different pathways, including PPARG, lipid, carbohydrate and energy metabolism, redox, membrane-organelle biosynthesis, and endocrine system. Downregulated genes were related to extracellular matrix and cell migration, proliferation, and differentiation. Analyses of mRNA-miRNA interaction showed that repressed miRNA-encoding genes can act downregulating PPARG-related genes; mostly the PPARG activator (PPARGC1A). Induced miRNA-encoding genes regulate downregulated genes related to TGFB1. These results shed new light to understand adipose-tissue differentiation and physiology, increasing our knowledge about pathologies like obesity, type-2 diabetes and osteoporosis. J. Cell. Physiol. 232: 771-784, 2017. © 2016 Wiley Periodicals, Inc.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Metabolic adaptations in the adipose tissue that underlie the body fat mass gain in middle-aged rats

Little is known about adipocyte metabolism during aging process and whether this can influence body fat redistribution and systemic metabolism. To better understand this phenomenon, two animal groups were studied: young-14 weeks old-and middle-aged-16 months old. Periepididymal (PE) and subcutaneous (SC) adipocytes were isolated and tested for their capacities to perform lipolysis and to incorporate D-[U-(14)C]-glucose, D-[U-(14)C]-lactate, and [9,10(n)-(3)H]-oleic acid into lipids. Additionally, the morphometric characteristics of the adipose tissues, glucose tolerance tests, and biochemical determinations (fasting glucose, triglycerides, insulin) in blood were performed. The middle-aged rats showed adipocyte (PE and SC) hypertrophy and glucose intolerance, although there were no significant changes in fasting glycemia and insulin. Furthermore, PE tissue revealed elevated rates (+50 %) of lipolysis during beta-adrenergic-stimulation. There was also an increase (+62 %) in the baseline rate of glucose incorporation into lipids in the PE adipocytes, while these PE cells were almost unresponsive to insulin stimulation and less responsive (a 34 % decrease) in the SC tissue. Also, the capacity of oleic acid esterification was elevated in baseline state and with insulin stimulus in the PE tissue (+90 and 82 %, respectively). Likewise, spontaneous incorporation of lactate into lipids in the PE and SC tissues was higher (+100 and 11 %, respectively) in middle-aged rats. We concluded that adipocyte metabolism of middle-aged animals seems to strongly favor cellular hypertrophy and increased adipose mass, particularly the intra-abdominal PE fat pad. In discussion, we have interpreted all these results as a metabolic adaptations to avoid the spreading of fat that can reach tissues beyond adipose protecting them against ectopic fat accumulation. However, these adaptations may have the potential to lead to future metabolic dysfunctions seen in the senescence.

Over-expression of TLR4-CD14, pro-inflammatory cytokines, metabolic markers and NEFAs in obese non-diabetic Mexicans

Introduction

Obesity is the world’s most important public health problem. Adipose tissue contributes significantly to increase pro-inflammatory mediators whose cascade begins with the union of TLR4 to its microbial ligands (TLR: Toll Like Receptors). It has been reported recently that NEFAs (Non-Esterified Fatty Acids) bind to this receptor as agonists. The purpose of our study was to determine the levels of expression of TLR4-CD14, the pro-inflammatory cytokines, the metabolic markers and the NEFAs in a group of adult, non-diabetic obese Mexicans.

Method

A group of 210 adult middle-class Mexican non-diabetic obese patients was evaluated: 105 normal weight individuals, and 105 non-diabetic obese. On both groups, the following was tested in each patient: TLR4-CD14 receptors on monocytes in peripheral blood, inflammatory profile, HOMA-IR (Homeostasis Model Assessment-Insulin Resistance), NEFAs and each individual’s anthropometric profile.

Results

Obesity is strongly associated with the expression of TLR4 (77%, MFI (Mean Fluorescence Index) 7.70) and CD14 (86% MFI 1.61) with 66% double positives (p = 0.000). These figures contrast with those for the normal weight individuals that constituted the control group: TLR4 (70% MFI 6.41) and CD14 (84% MFI 1.25) with 59% double positives. As for cytokine concentration, non- diabetic obese individuals vs the normal weight/thin, the numbers were: IL-1β = 2.0 vs 2.5 pg/ml (p = NS), IL-6 = 36 vs 28 pg/ml (p = 0.030), IL-8 = 27 vs 27 pg/ml (p = NS), IL-10 = 8.4 vs 6.8 pg/ml (p = NS), TNF-α =31 vs 15 pg/ml (p = 0.000) respectively. Insulin levels were 12.1 vs 19.7 mcU/ml (p = 0.000) and the NEFAs were much higher in the obese vs normal weight/thin individuals (p = 0.000).

Conclusion

Adipose tissue used to be thought of as mere storage of fats and energy, but it has been revealed to be an important neuro-immune-endocrine organ. Immune cells, stimulated by NEFAs, produce pro-inflammatory cytokines, which have a direct effect on oxidating radicals that directly target the release of noradrenalin. This in turn, reactivates the vicious cycle of low-grade chronic inflammation, as is now described in obesity.

Citrulline reduces glyceroneogenesis and induces fatty acid release in visceral adipose tissue from overweight rats

SCOPE

High-fat diet (HFD) increases visceral adipose tissue (AT). Our aim was to evaluate whether citrulline (CIT) affected nonesterified fatty acid (NEFA) metabolism in AT from HFD-fed rats.

METHODS AND RESULTS

Rats were fed for 8 weeks with either a control diet (CD) or HFD. Retroperitoneal AT explants were exposed to 2.5 mmol/L CIT for 24 h. We analyzed lipolysis, beta-oxidation, glyceroneogenesis, and the expression of the key associated enzymes. CIT doubled NEFA release selectively in HFD AT. Phosphorylation of hormone-sensitive lipase was upregulated 50 and 100% by CIT in CD and HFD AT, respectively. Under CIT, beta-oxidation increased similarly whatever the diet, whereas glyceroneogenesis, which permits NEFA re-esterification, was downregulated 50 and 80% in CD and HFD AT, respectively. In the latter, the important decrease in re-esterification probably explains the rise of NEFA release. A pretreatment with the nitric oxide synthase inhibitor N ω-nitro-l-arginine methyl ester abolished CIT effects.

CONCLUSION

These results demonstrate direct lipolytic and antiglyceroneogenic effects of CIT on CD and HFD AT. The selective CIT-mediated NEFA release from HFD AT was probably the consequence of the drastic decrease in glyceroneogenesis and nitric oxide was a mediator of CIT effects. These results provide evidence for a direct action of CIT on AT to reduce overweight.

Potential role of branched-chain amino acid catabolism in regulating fat oxidation

Insulin-resistant or obese individuals have increased serum branched-chain amino acid (BCAA) levels. Recent findings relate increased BCAA catabolism to increased fatty acid oxidation and better metabolic health in physically active individuals. We hypothesize that, via glyceroneogenesis, BCAA catabolism mediates increased constitutive use of fatty acids for β-oxidation in subjects with increased inherent or acquired aerobic capacity both during exercise and at rest.

Steroid-induced diabetes: a clinical and molecular approach to understanding and treatment

Since the advent of glucocorticoid therapy for autoimmune disease in the 1940s, their widespread application has led to the concurrent therapy-limiting discovery of many adverse metabolic side effects. Unanticipated hyperglycemia associated with the initiation of glucocorticoids often leads to preventable hospital admissions, prolonged hospital stays, increased risks for infection and reduced graft function in solid organ transplant recipients. Challenges in managing steroid-induced diabetes stem from wide fluctuations in post-prandial hyperglycemia and the lack of clearly defined treatment protocols. The mainstay of treatment is insulin therapy coincident with meals. This article aims to review the pathogenesis, risk factors, diagnosis and treatment principles unique to steroid-induced diabetes.

Principles of endocrinology

The endocrine system plays a major role in human survival. Endocrine glands secrete chemical messengers or hormones that affect every tissue of the body, including the periodontium, during the life of the individual. As the endocrine system influences a broad assortment of biological activities necessary for life, a general understanding of the principal components and functions of this system is essential. A fundamental assessment of hormone structure, mechanism of action and hormone transport, as well as influence on homeostasis is reviewed. A concise evaluation of the functions of the central endocrine glands, the functions of the major peripheral endocrine glands (other than gonadal tissues) and the known relationships of these hormones to the periodontium is examined.

Proteomic analysis of visceral adipose tissue in pre-obese patients with type 2 diabetes

The mechanisms involved in the progression to overt diabetes in pre-obese subjects remain unclear. Therefore, a nontargeted evaluation of differences in the protein abundance of visceral adipose tissue (VAT) obtained from pre-obese diabetic subjects and pre-obese subjects showing normal glucose tolerance may provide novel insights on the molecular processes involved in the progression to overt diabetes in pre-obesity. Diabetic patients showed increased VAT abundance of glutathione S-transferase Mu 2, peroxiredoxin-2, antithrombin-III, apolipoprotein A-IV, Ig κ chain C region, mitochondrial aldehyde dehydrogenase and actin, and decreased abundance of annexin-A1, retinaldehyde dehydrogenase-1, and vinculin, compared with their non-diabetic counterparts. These proteins are involved in cytoskeleton function and structure, oxidative stress, inflammation and retinoid metabolism. The presence of diabetes influences the VAT abundance of several proteins. Hence, the proteins identified here could be considered candidate molecules in future studies addressing the role that VAT dysfunction plays in the development of type 2 diabetes.

Skeletal muscle and beyond: the role of exercise as a mediator of systemic mitochondrial biogenesis

It has been known for more than 4 decades that exercise causes increases in skeletal muscle mitochondrial enzyme content and activity (i.e., mitochondrial biogenesis). Increasing evidence now suggests that exercise can induce mitochondrial biogenesis in a wide range of tissues not normally associated with the metabolic demands of exercise. Perturbations in mitochondrial content and (or) function have been linked to a wide variety of diseases, in multiple tissues, and exercise may serve as a potent approach by which to prevent and (or) treat these pathologies. In this context, the purpose of this review is to highlight the effects of exercise, and the underlying mechanisms therein, on the induction of mitochondrial biogenesis in skeletal muscle, adipose tissue, liver, brain, and kidney.

Long-term ritonavir exposure increases fatty acid and glycerol recycling in 3T3-L1 adipocytes as compensatory mechanisms for increased triacylglycerol hydrolysis

Lipodystrophy with high nonesterified fatty acid (FA) efflux is reported in humans receiving highly active antiretroviral therapy (HAART) to treat HIV infection. Ritonavir, a common component of HAART, alters adipocyte FA efflux, but the mechanism for this effect is not established. To investigate ritonavir-induced changes in FA flux and recycling through acylglycerols, we exposed differentiated murine 3T3-L1 adipocytes to ritonavir for 14 d. FA efflux, uptake, and incorporation into acylglycerols were measured. To identify a mediator of FA efflux, we measured adipocyte triacylglycerol lipase (ATGL) transcript and protein. To determine whether ritonavir-treated adipocytes increased glycerol backbone synthesis for FA reesterification, we measured labeled glycerol and pyruvate incorporation into triacylglycerol (TAG). Ritonavir-treated cells had increased FA efflux, uptake, and incorporation into TAG (all P < 0.01). Ritonavir increased FA efflux without consistently increasing glycerol release or changing TAG mass, suggesting increased partial TAG hydrolysis. Ritonavir-treated adipocytes expressed significantly more ATGL mRNA (P < 0.05) and protein (P < 0.05). Ritonavir increased glycerol (P < 0.01) but not pyruvate (P = 0.41), utilization for TAG backbone synthesis. Consistent with this substrate utilization, glycerol kinase transcript (required for glycerol incorporation into TAG backbone) was up-regulated (P < 0.01), whereas phosphoenolpyruvate carboxykinase transcript (required for pyruvate utilization) was down-regulated (P < 0.001). In 3T3-L1 adipocytes, long-term ritonavir exposure perturbs FA metabolism by increasing ATGL-mediated partial TAG hydrolysis, thus increasing FA efflux, and leads to compensatory increases in FA reesterification with glycerol and acylglycerols. These changes in FA metabolism may, in part, explain the increased FA efflux observed in ritonavir-associated lipodystrophy.

Lipolysis and the integrated physiology of lipid energy metabolism

Fat cell lipolysis, the cleavage of triglycerides and release of fatty acids and glycerol, evolved to enable survival during prolonged food deprivation but is paradoxically increased in obesity, in which a surfeit of all energy metabolites is found. Essential, previously-unsuspected components have been discovered in the lipolytic machinery, at the protective interface of the lipid droplet surface and in the signaling pathways that control lipolysis. At least two adipocyte lipases are important for controlling lipolysis, hormone-sensitive lipase (HSL) and adipocyte triglyceride lipase (ATGL). Perilipin (PLIN) and possibly other proteins of the lipid droplet surface are master regulators of lipolysis, protecting or exposing the triglyceride core of the droplet to lipases. The prototypes for hormonal lipolytic control are beta adrenergic stimulation and suppression by insulin, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and PLIN. Newly-recognized mediators of lipolysis include atrial natriuretic peptide, cyclic GMP, the ketone body 3-hydroxybutyrate, AMP kinase and mitogen-activated kinases. Lipolysis must be interpreted in its physiological context since similar rates of basal or stimulated lipolysis occur under different conditions and by different mechanisms. Age, sex, anatomical site, genotype and species differences are each important variables. Manipulation of lipolysis has therapeutic potential in several inborn errors and in the metabolic syndrome that frequently complicates obesity.

Long-term pioglitazone treatment enhances lipolysis in rat adipose tissue

OBJECTIVES

The insulin-sensitizing effects of thiazolidinediones are believed to depend at least in part on reductions in circulating levels of nonesterified fatty acids (NEFA). The mechanisms that mediate the reductions in NEFA are not fully understood and could involve reductions in adipose tissue lipolysis, increases in glyceroneogenesis and NEFA reesterification in triglycerides in adipose tissue and increases in NEFA metabolism by oxidative tissues.

METHODS

In a congenic strain of spontaneously hypertensive rats that fed a high-sucrose diet to promote features of the metabolic syndrome, we studied the effects of chronic pioglitazone treatment over 4 months on adipose tissue lipolysis and NEFA metabolism.

RESULTS

We observed significant increases in basal and adrenaline-stimulated NEFA and glycerol release, and near-total suppression of NEFA reesterification in epididymal adipose tissue isolated from rats chronically treated with pioglitazone. However, pioglitazone-treated rats also exhibited significant increases in mitochondrial DNA levels in adipose tissue (3.2-fold increase, P=0.001) and potentially greater sensitivity to the antilipolytic effects of insulin than untreated controls. In addition, chronic pioglitazone treatment was associated with increased palmitate oxidation in soleus muscle, reduced fasting levels of serum NEFA and triglycerides, as well as reduced serum levels of insulin and increased serum levels of adiponectin.

CONCLUSIONS

Despite suppressing NEFA reesterification and increasing basal and adrenaline-stimulated lipolysis, chronic pioglitazone treatment may decrease circulating NEFA levels in part by increasing adipose tissue sensitivity to the antilipolytic effects of insulin and by enhancing NEFA oxidation in skeletal muscle.

Glycerolipid metabolism and signaling in health and disease

Maintenance of body temperature is achieved partly by modulating lipolysis by a network of complex regulatory mechanisms. Lipolysis is an integral part of the glycerolipid/free fatty acid (GL/FFA) cycle, which is the focus of this review, and we discuss the significance of this pathway in the regulation of many physiological processes besides thermogenesis. GL/FFA cycle is referred to as a "futile" cycle because it involves continuous formation and hydrolysis of GL with the release of heat, at the expense of ATP. However, we present evidence underscoring the "vital" cellular signaling roles of the GL/FFA cycle for many biological processes. Probably because of its importance in many cellular functions, GL/FFA cycling is under stringent control and is organized as several composite short substrate/product cycles where forward and backward reactions are catalyzed by separate enzymes. We believe that the renaissance of the GL/FFA cycle is timely, considering the emerging view that many of the neutral lipids are in fact key signaling molecules whose production is closely linked to GL/FFA cycling processes. The evidence supporting the view that alterations in GL/FFA cycling are involved in the pathogenesis of "fatal" conditions such as obesity, type 2 diabetes, and cancer is discussed. We also review the different enzymatic and transport steps that encompass the GL/FFA cycle leading to the generation of several metabolic signals possibly implicated in the regulation of biological processes ranging from energy homeostasis, insulin secretion and appetite control to aging and longevity. Finally, we present a perspective of the possible therapeutic implications of targeting this cycling.

Effects of ritonavir on adipocyte gene expression: evidence for a stress-related response

To understand the molecular mechanisms underlying the development of dyslipidemia and lipodystrophy that occurs after administration of aspartic acid protease inhibitors, we examined transcriptional profiles using cDNA microarrays in 3T3-L1 adipocytes exposed to 10 micromol/l ritonavir for 2-21 days. The expression levels of approximately 12,000 transcripts were assessed using the MgU74Av2 mouse microarray chip. Ritonavir altered gene expression of inflammatory cytokines, stress response genes localized to endoplasmic reticulum, oxidative stress genes, apoptosis-related genes, and expression of genes involved in cell adhesion and extracellular matrix remodeling. Microarray analysis also identified a novel gene downregulated by ritonavir, Cidea, whose expression levels may affect free-fatty acid metabolism. These changes suggest a unique, stress-related pattern in adipocytes induced by chronic exposure to the protease inhibitor, ritonavir.

Monocyte chemoattractant protein-1 secreted by adipose tissue induces direct lipid accumulation in hepatocytes

For many years, adipose tissue has been mainly considered as an inert reservoir for storing triglycerides. Since the discovery that adipocytes may secrete a variety of bioactive molecules (hormones, chemokines, and cytokines), an endocrine and paracrine role for white adipose tissue (WAT) in the regulation of energy balance and other physiological processes has been established, particularly with regard to brain and muscle. In contrast, little is known about the interactions of WAT with liver. Hence, we examined the effect of the secretory products of WAT on hepatocytes. Conditioned medium of human WAT explants induced significant steatosis in hepatocyte cell lines. Factor(s) responsible for the conditioned medium-induced steatosis were screened by a battery of blocking antibodies against different cytokines/chemokines shown to be secreted by WAT. In contrast to interleukin-8 and interleukin-6, the monocyte chemoattractant protein-1 was capable of inducing steatosis in hepatocytes in a time-dependent manner at concentrations similar to those found in conditioned medium. Incubation of conditioned medium with antimonocyte chemoattractant protein-1 antibodies prevented triglyceride accumulation. Investigation of the mechanism leading to the triglyceride accumulation showed that both a diminution of apolipoprotein B secretion and an increase in phosphoenolpyruvate carboxykinase messenger RNA may be involved.

CONCLUSION

The monocyte chemoattractant protein-1 secreted by adipose tissue may induce steatosis not only recruiting macrophages but also acting directly on hepatocytes.

Retinoids upregulate phosphoenolpyruvate carboxykinase and glyceroneogenesis in human and rodent adipocytes

Glyceroneogenesis is an important metabolic pathway for fatty acid reesterification in adipose tissue, thereby reducing fatty acid release. Glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), which is the key enzyme in this pathway, are both regulated by a series of hormones and nutrients, among which all-trans retinoic acid (all-trans RA) is a transcriptional inducer of the PEPCK-C gene (Pck1). All-trans RA binds to the retinoic acid receptor (RAR) and activates it, whereas its stereoisomer 9-cis retinoic acid (9-cis RA) is a ligand for the 9-cis RA receptor (RXR). Three RXR-binding elements [retinoic acid response element (RARE)1/PCK1, RARE2, and RARE3/PCK2] were previously located in the promoter of Pck1. Using 3T3-F442A adipocytes, we demonstrated that Pck1 expression was 10-fold more sensitive to 9-cis RA (EC(50): 10 nmol/L) than to all-trans RA. We then analyzed the respective involvement of RARE1/PCK1, RARE2, and RARE3/PCK2 in the response of Pck1 to 9-cis RA and all-trans RA in adipocytes. The response to 9-cis RA mainly involved the RARE1/PCK1 element, whereas RARE2 was mainly responsive to all-trans RA. In contrast, the full response to both RA isomers involved these 2 elements and included RARE3/PCK2 as well. Furthermore, 9-cis RA, but not all-trans RA, selectively induced PCK1 in ex-vivo-treated human adipose tissue explants, with a concomitant induction of glyceroneogenesis monitored by [1-(14)C]-pyruvate incorporation into neutral lipids. The concomitant 9-cis RA-induced reduction in fatty acid output indicates an important role for this RA stereoisomer in lipid homeostasis through stimulation of PEPCK-C and glyceroneogenesis in adipose tissue.

Free fatty acids as mediators of adaptive compensatory responses to insulin resistance in dexamethasone-treated rats

BACKGROUND

Chronic low-dose dexamethasone (DEX) treatment in rats is associated to insulin resistance with compensatory hyperinsulinaemia and reduction in food intake. We tested the hypothesis that the elevation in circulating free fatty acids (FFAs) induced by DEX is the common mediator of both insulin resistance and insulin hyperproduction.

METHODS

For this purpose, an anti-lipolytic agent was administered during DEX treatment to lower lipacidaemia for several hours prior to glucose and insulin tolerance tests. Leptin expression in adipose tissue (by Northern blot) and plasma leptin levels (by radioimmunoassay) were also investigated to verify whether a rise in circulating leptin could be responsible for the anorectic effect of DEX.

RESULTS

Our data show that a transient pharmacological reduction of elevated plasma FFA levels abates the post-loading hyperinsulinaemia and counteracts the insulin resistance induced by DEX, supporting the hypothesis that the chronic elevation in FFAs is the common mediator of DEX-induced changes. Despite enhanced leptin expression in white adipose tissue, DEX-treated rats show no significant increase in plasma leptin levels. This suggests that the anorectic effect of DEX should be mediated, at least partially, by other factors, possibly related to the influence of concomitantly elevated plasma FFA and insulin levels on the hypothalamic centers regulating feeding.

CONCLUSIONS

Our results sustain the idea that a prolonged increase in plasma FFA levels plays an important role in the adaptive regulation of glucose and energy homeostasis, not only by potentiating insulin secretion but also by providing a signal of 'nutrient abundance' capable of restraining food intake.

Adipose tissue integrity as a prerequisite for systemic energy balance: a critical role for peroxisome proliferator-activated receptor gamma

Peroxisome proliferator-activated receptor gamma (PPARgamma) is an essential regulator of adipocyte differentiation, maintenance, and survival. Deregulations of its functions are associated with metabolic diseases. We show here that deletion of one PPARgamma allele not only affected lipid storage but, more surprisingly, also the expression of genes involved in glucose uptake and utilization, the pentose phosphate pathway, fatty acid synthesis, lipolysis, and glycerol export as well as in IR/IGF-1 signaling. These deregulations led to reduced circulating adiponectin levels and an energy crisis in the WAT, reflected in a decrease to nearly half of its intracellular ATP content. In addition, there was a decrease in the metabolic rate and physical activity of the PPARgamma(+/-) mice, which was abolished by thiazolidinedione treatment, thereby linking regulation of the metabolic rate and physical activity to PPARgamma. It is likely that the PPARgamma(+/-) phenotype was due to the observed WAT dysfunction, since the gene expression profiles associated with metabolic pathways were not affected either in the liver or the skeletal muscle. These findings highlight novel roles of PPARgamma in the adipose tissue and underscore the multifaceted action of this receptor in the functional fine tuning of a tissue that is crucial for maintaining the organism in good health.

Acute and selective inhibition of adipocyte glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase by interferon gamma

Interferon gamma (IFN-gamma) was previously shown to promote fatty acid (FA) release from adipose tissue (AT). Net lipolysis is an equilibrium between triglyceride breakdown and FA re-esterification. The latter requires activated glyceroneogenesis for glycerol-3-phosphate synthesis and increased cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), the key enzyme in this pathway. We wondered whether glyceroneogenesis and PEPCK-C would be IFN-gamma targets. We injected mice with IFN-gamma, and exposed either AT explants and isolated adipocytes from humans and mice or 3T3-F442A adipocytes to IFN-gamma before monitoring expression of genes involved in lipid metabolism and the metabolic consequences. We show that IFN-gamma induces a large increase in FA release without affecting glycerol output and decreases [1-(14)C]-pyruvate incorporation into lipids, thus demonstrating that FA re-esterification is reduced due to diminished glyceroneogenesis. A series of mRNA encoding proteins involved in FA metabolism remained unaffected by IFN-gamma, while that of PEPCK-C was rapidly and drastically lowered. IFN-gamma effect opposed that of the beta-agonist isoproterenol and of 8-Br-cAMP. In IFN-gamma-treated mice, PEPCK-C gene expression was decreased in AT, but not in liver or kidney. Thus, IFN-gamma exerts a tissue-specific action in rodents and humans, having glyceroneogenesis and the PEPCK-C gene as selective targets to intensify FA release from adipocytes.

Acute and selective regulation of glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase in adipose tissue by thiazolidinediones in type 2 diabetes

AIMS/HYPOTHESIS

Regulation of glyceroneogenesis and its key enzyme cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) plays a major role in the control of fatty acid release from adipose tissue. Here we investigate the effect of rosiglitazone on the expression of genes involved in fatty acid metabolism and the resulting metabolic consequences.

MATERIALS AND METHODS

Rosiglitazone was administered to Zucker fa/fa rats for 4 days and to 24 diabetic patients for 12 weeks, then mRNA expression for the genes encoding PEPCK-C, mitochondrial PEPCK, adipocyte lipid-binding protein, glycerol kinase, lipoprotein lipase and glycerol-3-phosphate dehydrogenase was examined in s.c. adipose tissue by real-time RT-PCR. Glyceroneogenesis was determined using [1-(14)C]pyruvate incorporation into lipids. Cultured adipose tissue explants from overweight women undergoing plastic surgery were incubated with rosiglitazone for various times before mRNA determination and analysis of PEPCK-C protein, activity and glyceroneogenesis.

RESULTS

Rosiglitazone administration to rats induced the expression of the gene encoding PEPCK-C mRNA (PCK1) and PEPCK-C activity in adipose tissue with a resulting 2.5-fold increase in glyceroneogenesis. This was accompanied by an improvement in dyslipidaemia as demonstrated by the decrease in plasma NEFAs and triacylglycerol. In rosiglitazone-treated diabetic patients, PCK1 mRNA was raised 2.5-fold in s.c. adipose tissue. Rosiglitazone treatment of adipose tissue explants from overweight women caused a selective augmentation in PCK1 mRNA which reached a maximum of 9-fold at 14 h, while mRNA for other genes remained unaffected. Experiments with inhibitors showed a direct and transcription-only effect, which was followed by an increase in PEPCK-C protein, enzyme activity and glyceroneogenesis.

CONCLUSIONS/INTERPRETATION

These results favour adipocyte glyceroneogenesis as the initial thiazolidinedione-responsive pathway leading to improvement in dyslipidaemia.